1. Field of the Invention
This invention relates generally to a method for forming and exploiting gradients at the interfaces between components of a composite material, as well as to the composite material itself, and devices which incorporate the material. In particular, the invention relates to a method for forming and exploiting magnetic gradients at the interfaces between components of a magnetic composite material and the magnetic composite material itself as well as devices which incorporate the composite material such as electrochemical systems and separators including fuel cells, batteries, and separations resulting in enhanced and modified flux and performance in those systems. The invention further relates to compositions, apparatus, methods of making, and methods of using magnetic composite materials in electrolytic applications, including fuel cells. In particular, the invention relates to compositions, apparatus, methods of making, and methods of using magnetic composite materials in electrolytic applications to prevent electrode passivation, particularly in fuel cell applications for the direct reformation of fuels; and to apparatus and methods for modifying the outcome of electrolyses involving free radical products and intermediates.
As used herein, the term "fuel" includes mixtures of one or more fuels, either liquid or gaseous, with other fuel or non-fuel components, including fuel mixtures of one or more fuels with air. As used herein, the term "fuel mixture" refers to a mixture of a fuel with one or more different fuel, or non-fuel, components.
2. Background of the Related Art
In the detailed description of preferred embodiments, it will be shown that interfacial gradients in properly prepared composite materials can be exploited to enhance flux in many types of electrochemical systems such as fuel cells, batteries, membrane sensors, filters and flux switches. Such interfacial gradients may also be exploited in separators involving chromatographic separations and nonelectrochemical separations including, but not limited to, separations of light and heavy transition metals and transition metal complexes. The heavy transition metals include the lanthanides and the actinides which have atomic numbers 58-71 and 90-103, respectively. First, however, the following discussion provides a brief overview of the current understanding of magnetic properties in composites. In particular, the discussion below summarizes the thermodynamic, kinetic and mass transport properties of bulk magnetic materials. These bulk properties of molecules in magnetic fields are fairly well understood.
Rudimentary Magnetic Concepts
Paramagnetic molecules have unpaired electrons and are attracted into a magnetic field; diamagnetic species, with all electrons paired, are slightly repelled by the field. Radicals and oxygen are paramagnetic; most organic molecules are diamagnetic; and most metal ions and transition metal complexes are either para- or diamagnetic. How strongly a molecule or species in a solution or fluid responds to a magnetic field is parameterized by the molar magnetic susceptibility, .chi..sub.m (cm.sup.3 /mole). For diamagnetic species, .chi..sub.m is between (-1 to -500).times.10.sup.-6 cm.sup.3 /mole, and temperature independent. For paramagnetic species, .chi..sub.m ranges from 0 to +0.01 cm.sup.3 /mole, and, once corrected for its usually small diamagnetic component, varies inversely with temperature (Curie's Law).
While ions are monopoles and will either move with or against an electric field., depending on the sign of the ion, paramagnetic species are dipoles and will always be drawn into (aligned in) a magnetic field, independent of the direction of the magnetic vector. These dipoles will experience a net magnetic force if a field gradient exists. Because electrochemistry tends to involve single electron transfer events, the majority of electrochemical reactions should result in a net change in the magnetic susceptibility of species near the electrode.
Magnetic field effects on chemical systems can be broken down into three types: thermodynamic, kinetic, and mass transport. Macroscopic, thermodynamic effects are negligible, although local magnetic field effects may not be. Kinetically, both reaction rates and product distributions can be altered. Transport effects can lead to flux enhancements of several-fold. Quantum mechanical effects are also possible, especially on very short length scales, below 10 nm. The following summarizes what has been done with homogeneous fields applied to solutions and cells with external laboratory magnets.
Thermodynamics
A magnetic field applied homogeneously by placing a solution between the poles of a laboratory magnet will have a negligible nonexponential effect on the free energy of reaction. .DELTA.G.sub.m =-0.5.DELTA..chi..sub.m B.sup.2 J/mole, where .DELTA.G.sub.m is the change of the free energy of reaction due to the magnetic field, .DELTA..chi..sub.m is the difference in magnetic susceptibility of the products and reactants, and B is the magnetic induction in gauss. For the conversion of a diamagnetic species into a paramagnetic species, .DELTA..chi..sub.m.ltoreq.0.01 cm.sup.3 /mole. In a 1 Tesla (T) (1 Tesla=10 kGauss) applied field, .vertline..DELTA.G.sub.m.vertline..ltoreq.0.05 J/mole. Even in the strongest laboratory fields of 10 T, the effect is negligible compared to typical free energies of reaction (.congruent.kJ/mole). These are macroscopic arguments for systems where the magnet is placed external to the cell and a uniform field is applied to the solution. Microscopically, it may be possible to argue that local fields in composites are substantial, and molecules in composites within a short distance of the source of the magnetic field experience strong local fields. For example, for a magnetic wire or cylinder, the magnetic field falls off over a distance, x, as x.sup.-3. The field experienced by a molecule 1 nm from the magnet is roughly 10.sup.21 times larger than the field experienced at 1 cm. This argument is crude, but qualitatively illustrates the potential advantage of a microstructural magnetic composite. (As an example, in the magnetic/Nafion (DuPont) composites, a larger fraction of the redox species are probably transported through the 1.5 nm zone at the interface between the Nafion and the magnetic particles.) These redox species must therefore experience large magnetic fields in close proximity to the interface.
Kinetics
Reaction rates, k, are parameterized by a pre-exponential factor, A, and a free energy of activation, .DELTA.G.sup..dagger-dbl. ; k=A exp[-.DELTA.G.sup..dagger-dbl. /RT]. An externally applied, homogeneous magnetic field will have little effect on .DELTA.G.sup..dagger-dbl., but can alter A. Nonadiabatic systems are susceptible to field effects. Magnetic fields alter the rate of free radical singlet-triplet interconversions by lifting the degeneracy of triplet states (affecting .DELTA.G.sup..dagger-dbl.); rates can be altered by a factor of three in simple solvents. Because magnetic coupling occurs through both electronic nuclear hyperfine interactions and spin-orbit interactions, rates can be nonmonotonic functions of the applied field strength. Photochemical and electrochemical luminescent rates can be altered by applied fields. For singlet-triplet interconversions, magnetic fields alter product distributions when they cause the rate of interconversion to be comparable to the rate at which free radicals escape solvent cages. These effects are largest in highly viscous media, such as polymer films and micellar environments. Larger effects should be observed as the dimensionality of the system decreases. For coordination complexes, photochemical and homogeneous electron transfer rates are altered by magnetic fields. Spin-orbit coupling is higher in transition metal complexes than organic radicals because of higher nuclear charge and partially unquenched orbital angular momentum of the d- or f-shell electrons. The rate of homogeneous electron transfer between Co(NH.sub.3).sub.6.sup.3+ and Ru(NH.sub.3).sub.6.sup.2+ is below that expected for diffusion controlled reactions; in a 7 T magnetic field, the rate is suppressed two to three-fold. It has been argued that .DELTA..chi..sub.m (and .DELTA.G.sub.m) is set by the magnetic susceptibility of the products, reactants, and activated complex, and a highly paramagnetic activated complex accounts for the field effect. For reversible electron transfer at electrodes in magnetic fields, no significant effect is expected. For quasireversible electron transfer with paramagnetic and diamagnetic species, electron transfer rates and transfer coefficients (.alpha.) are unchanged by magnetic fields applied parallel to electrodes. Magnetic fields applied perpendicular to electrodes in flow cells generate potential differences, which just superimpose on the applied electrode potentials. Potentials of 0.25V have been reported. Reversing the applied magnetic field reverses the sign of the potential difference. This effect does not change standard rate constants, only the applied potential.
Mass Transport
Magnetically driven mass transport effects have been studied in electrochemical cells placed between the poles of large magnets. Effects vary depending on the orientation of the electrode, the relative orientation of the magnetic field and the electrode, forced or natural convection, and the relative concentrations of the redox species and electrolyte. Three cases are illustrated in FIGS. 1, 2 and 3.
For a charged species moving by natural or forced convection parallel to an electrode and perpendicular to a magnetic field which is also parallel to the electrode, a Lorentz force is generated which moves the charged particle toward the electrode (FIG. 1). This magnetohydrodynamic effect is characterized by EQU F=q(E+v.times.B) (1)
where F, E, v, and B are vectors representing the Lorentz force on the charged species, the electric field, the velocity of the moving species, and the magnetic field, respectively; and q is the charge on the species. For flow cells and vertical electrodes, flux enhancements of a few-fold and reductions in the overpotential of a few tenths volts have been found in the presence of the magnetic field. Also, embedded in Equation 1 is the Hall effect; when a charged species moves through a perpendicular magnetic field, a potential is generated. This potential superimposes on the applied potential and causes migration in low electrolyte concentrations.
When the electrode and magnetic field are parallel to the earth, thermal motion leads to vortical motion at the electrode surface unless the field (B) and the current density (j) are spatially invariant and mutually perpendicular (see FIG. 2). This is parameterized as: EQU F.sub.v =c.sup.-1 [j.times.B] (2)
In Equation (2) F.sub.v is the vector of magnetic force per volume and c is the speed of light. In general, these forces are smaller than Lorentz forces; flux enhancements of a few-fold and potential shifts of 10 to 20 mV are observed. Flux enhancements of paramagnetic and diamagnetic species are similar, but paramagnetic electrolytes enhance the flux of diamagnetic Zn.sup.2+ two-fold. Vortices suppress thermal motion and eddy diffusion.
The final configuration, shown in FIG. 3, is for the magnetic field perpendicular to the electrode surface, and, therefore, parallel to the electric field. Various, and sometimes inconsistent, results are reported for several configurations: for vertical electrodes in quiescent solution, flux enhancements of .ltoreq.1000%; for electrodes parallel to the earth with forced convection, flux retardations of 10%; and for electrodes parallel to the earth and no forced convection, both enhancements and no enhancements are reported.
The above summarizes the thermodynamic, kinetic, and mass transport effects for systems where the magnetic field is applied uniformly across a cell with an external magnet. None of these macroscopic effects predict or address properties dependent on the magnetic susceptibility of the redox species. Quantum mechanical effects may also be important, especially on short length scales.
Fuel Cells
Since the incomplete reduction of oxygen limits the efficiency of H.sub.2 /O.sub.2 solid polymer electrolyte fuel cells, the cathode must be pressurized about five-fold over the anode.
Proton exchange membrane (PEM) fuel cell design is one which employs hydrogen as an anode feed and oxygen in air as a cathode feed. These fuels are decomposed electrolytically (to yield water) at electrodes typically modified with a noble metal catalyst. The hydrogen and oxygen are separated from each other by a proton exchange membrane (such as Nafion) to prevent thermal decomposition of the fuels at the noble metal catalyst. The reactions at the cathode can be summarized as follows: ##EQU1##
However, the fuel cell is typically run under non-equilibrium conditions, and, as such, is subject to kinetic limitations. These limitations are usually associated with the reaction at the cathode. EQU O.sub.2 +4H.sup.+ +4e=2H.sub.2 O E.degree..sub.cathode =1.23V
As the reaction at the cathode becomes increasingly kinetically limited, the cell voltage drops, and a second reaction path, the two electron/two proton reduction of oxygen to peroxide, becomes increasingly favored. This consumes oxygen in two electron steps with lower thermodynamic potential. EQU O.sub.2 +2H.sup.+ +2e=H.sub.2 O.sub.2 E.degree.H.sub.2 O.sub.2 =0.68V
The standard free energy of this reaction is 30% of the free energy available from the four electron reduction of oxygen to water. The decrease in current associated with the decreased number of electrons transferred and the decreased cell potential couple to yield substantially lower fuel cell power output.
One approach to enhance the efficiency of the cathodic reaction is to increase the concentration (pressure) of the feeds to the cathode--protons and oxygen--so as to enhance the flux (i.e., the reaction rate at the cathode in moles/cm.sup.2 s.sup.-1) at the cathode. The proton flux is readily maintained at a sufficiently high value by the proton exchange membrane (usually Nafion) so as to meet the demand set by the cathode reaction. Normally, the method of enhancing the flux and biasing the reaction to favor the formation of water is to pressurize the air feed to the cathode. Pressures of 5-10 atmospheres are typical.
The need to pressurize air to the cathode in PEM fuel cells has been a major obstacle in the development of a cost effective fuel cell as a replacement for the internal combustion engine, e.g. in a vehicle. In particular, pressurization of the cathode requires compressors. In transportation applications, power from the fuel cell is needed to run the compressor. This results in approximately 15% reduction in the power output of the total fuel cell system.
By developing a passive pressurization method for a fuel cell, the mechanical pumps could be eliminated from the fuel cell system. This has numerous advantages. The weight of a fuel cell would be decreased almost 40% by eliminating mechanical pumps. With the elimination of the parasitic loss of running the pumps, and improving cathode performance, the fuel cells would provide a higher energy and power density. Any potential shift at the electrode surface driven by the magnetic components can be exploited to enhance the voltage output of the fuel cell, and to overcome the poor kinetics of the cathode. Eliminating the pumps also eliminates the only moving parts of the fuel cell, and thereby, the likelihood of fuel cell failure is drastically reduced.
In current fuel cell design, the cathode is pressurized to approximately five times the pressure of the anode. This pressurization constrains the design of the fuel cell to be sufficiently rigid as to support this pressure. In a magnetically based, passive pressurization scheme, the need for the rigid structure is eliminated. This has two major advantages. First, the weight and bulk of the fuel cell is decreased. Second, the fuel cell is now a flexible device. The flexibility can be exploited in various ways, including placing fuel cells into unusual geometries and structures, thereby exploiting space in structures and devices which might otherwise be lost. Also, the flexible nature of the fuel cell allows a single structure to be divided readily into several smaller cells, which can be connected into different parallel and serial configurations to provide variable voltage and current outputs. Such a division is more complicated in the more rigid structures of a pressurized fuel cell because the encasing walls limit access to the fuel cell electrodes.
Electrode Passivation
At present, many fuel cell designs are focused on hydrogen as fuel, due in part to the favorable kinetics for hydrogen at the anode. However, hydrogen has the disadvantage of being difficult and somewhat dangerous to store. This disadvantage is particularly apparent in portable fuel cell applications (e.g. vehicular and portable device applications, such as lap-top computers). For vehicular fuel cell applications a liquid fuel, such as methanol, is generally preferred. In prior art fuel cells employing methanol as fuel, methanol is reformed thermally over a copper zinc or other catalyst to form hydrogen (and carbon dioxide), which is fed to the anode on demand.
Direct reformation of liquid fuels (e.g. methanol, ethanol) at the anode, although much preferred, is not efficient using prior art fuel cells. This is so because, in prior art fuel cells, a passivating layer rapidly forms on the surface of the anode due to reaction of the anode surface with intermediates of the electrolysis, i.e., the electrode is rendered passive, and as a result further reaction stops or is significantly impeded.
Despite extensive investigations over the past several decades, little progress has been made in developing direct reformation of a fuel at the electrode(s) of a fuel cell. This is particularly true in the case of direct reformation at ambient temperatures. This lack of progress is a direct consequence of the lack of resolution of the problem of electrode passivation. To date, almost all efforts to resolve this problem have focused on chemical techniques and methods, as opposed to the physical/magnetic approach adopted under the present invention.
Data presented below demonstrate that electrodes modified with a magnetic composite are either not passivated or passivated less than unmodified electrodes, i.e. such modified electrodes resist passivation; whereas unmodified electrodes are rapidly passivated, during oxidation of a liquid fuel. According to the present invention, the direct reformation of liquid fuel in fuel cell applications is possible. The present invention may also be broadly applicable to the prevention of passivation in the case of all electrolyses which proceed via either a free radical mechanism, or a multi-electron transfer process.